Sunday, October 21, 2012

The journey

I have lately signed up to participate in the Genographic Project. This remarkable effort by the National Geographic Society “is an ambitious attempt to help answer fundamental questions about where we originated and how we came to populate the earth.” All you need to do is buy the kit, follow the instructions, take a swab, post it off to the lab, and wait patiently.

When DNA (Deoxyribonucleic acid) is passed from one generation to the next, most of it is recombined by processes that endow each of us with our individuality. But some portions of DNA remain largely intact through the generations, altered only occasionally by spontaneous mutations, the equivalent of a genetic spelling or typographical error.


Such mutations survive in the form of genetic markers, of which approximately 150,000 have now been reliably isolated. These markers allow geneticists to map with extraordinary precision our common evolutionary development through innumerable generations, and to all the different regions through which man has gradually migrated (or not) since we originated with a single ancestor who lived in Africa approximately 140,000 years ago.


This in turn has made possible the study of genetic anthropology, whose value, among many other things, is to impress upon all people everywhere the shared aspects of our descent from that common ancestor, and even the extent to which we preserve within ourselves a measure of contact between some of our most distant ancestors and now extinct hominid cousins in the Neanderthal and Denisovan families. There is also the advantage of building a sufficiently large database that will help medical scientists to get to the bottom of certain diseases for which there may be some not yet sufficiently understood genetic predisposition.

According to the Project website, the human body is composed of trillions of cells, which form the basic units of life and combine to form more complex tissues and organs. Inside each cell, genes make up a sort of blueprint for protein production that determines how each will function. Genes also determine physical characteristics or traits. The complete set of some 20,000 to 25,000 genes is called the genome. Only a tiny fraction of the genome sets the human body apart from those of other animals, which, incidentally, is another reason why the Genographic Project is an important one to participate in. We are not merely urged to see ourselves as distant cousins of all other people, but as far more closely related to many other forms of life on earth than we have ever before been inclined fully to grasp.

Mostly half of the genes that make up our genome came from our father, and half from our mother. Each half represents a shuffled combination of DNA passed down to us from our ancestors. This process of shuffling, recombination, and shuffling again, generation after generation, makes it impossible to trace lines of descent because it creates an ever new genetic mixture of all our ancestors.

Fortunately for anthropological geneticists, however, there are parts of the genome that are passed intact from parent to child, and have not been shuffled at all. In these segments the genetic code varies only when from time to time spontaneous mutations occur at random.

The Y chromosome is the sex-determining chromosome in humans. While all other chromosomes are found in matching pairs, it is the mismatch of the Y chromosome with its partner, the X chromosome, that determines gender—men have a mismatched pair (Y and X), while women have two X chromosomes. Because the Y chromosome does not have a matching chromosome, most of it escapes the shuffling process. The Y chromosome passes intact from father to son, altered only by occasional mutations.

The mitochondrial genome is the female counterpart of the Y chromosome. Mitochondria are self-reproducing structures found inside the cells of all higher organisms, typically present in hundreds of copies per cell. They are responsible for generating most of the energy used by the cell. Because there are no mitochondria in the head of a mature sperm, they are passed down solely from mother to child, and thenceforth through successive generations of daughters, granddaughters, and so on.

One region of particular importance in mitochondrial DNA is the so-called hypervariable region, where the pace of mutation is up to a hundred times faster than that of the nuclear genome. Because of its much shorter length (several hundred nucleotides versus millions of nucleotides for the Y chromosome), the hypervariable region can be scanned quickly and reveals many highly informative mutational events that have been passed down through the matriline.

So we have much to learn from the Y chromosome DNA we inherited from Dad and the mitochondrial DNA from Mum, but the exact shape of what is effectively an immense whole-of-population family tree is further shaped to a larger or lesser degree by evolutionary forces, most importantly natural selection, migration, and what is known as genetic drift.

Except for religious zealots, everyone outside certain of the United States of America knows what natural selection is, and how Charles Darwin and others discovered how it works to preserve and refine beneficial genetic mutations and to eliminate harmful ones—insuring the survival of the fittest, although evidently this does not apply to those males of the species who currently run the international financial and investment banking sectors. Likewise, the impact of vastly different natural environments through which humans have migrated—hot ones, cold ones, wet ones, dry ones, mountainous ones, or marshy ones—have had an obvious impact upon the mechanisms of natural selection in different places and through different ages.

However, some genetic changes, such as “allele frequency,” occur randomly within discrete sub-populations and are duly passed from parent to child—thus imposing a further shaping influence upon human evolution. Allele frequency is the measurable change in the frequency of a gene variant or allele in populations due to random sampling. The effect of this genetic drift varies sharply with the size of sub-populations. Small sub-populations confined for very long periods to a distant and rarely visited island, for example—think Iceland, think Tasmania, think Madagascar—are subject to much greater genetic drift for the same reason that scoring seven heads and three tails after ten coin tosses is far, far more likely to occur than scoring 700,000 heads after one million tosses.

Fascinating, yes, but what does it all add up to?

Approximately 60,000 years ago homo sapiens very nearly became extinct, as eventually the Neanderthals and Denisovans did, though not before successfully mating with our ancestors when they encountered one another in Eurasia not too long afterwards. (Those of us who stayed put in sub-Saharan Africa never had this opportunity.) It is worth repeating: We almost vanished from the face of the earth before we even started our common journey; it was a very close call. 

At that time, pressed by some combination of presumably terrible circumstances, groups separating from as few as 2,000 surviving specimens of homo sapiens set out from the Asaf Depression in Ethiopia: north, west, and south. 

By 50,000 years ago, our ancestors had successfully multiplied, pulling back from the brink of extinction, fanning across south Asia and across the land bridge into Australia. By 40,000 years ago we had pushed into Mesopotamia and through Southeast Asia and up into China. By 30,000 years ago we had crossed back over Central Asia, and by that route into Northern Europe. At around the same time some of us double-backed from Central Asia and headed down into the Indian subcontinent. 20,000 years ago there was a great migration from East Asia over another land bridge into North America and ownards into South America. 

10,000 years ago further migrations occurred from the Middle East through southern Europe and across North Africa, in other words around both sides of the Mediterranean Sea; from Southeast Asia into what turned into the Indonesian archipelago, and a fresh wave from Northeast Asia into North America. These most recent global shifts in population appear to have coincided with the invention of agriculture, though there is mounting evidence that much earlier fire was successfully manipulated to “manage” natural environments, especially in Australia. 

We do not know when or how language gradually developed over these massively long periods, but it was probably very early. We mastered the horse in approximately 4,000 to 3,500 B.C. Certainly writing and numbers were invented soon afterwards in Mesopotamia, in approximately 3,200 B.C., and, quite separately, in Central America in about 600 B.C. Most intellectual, philosophical, scientific, artistic and many other attainments have occurred since then, barely 5,500 years in a 60,000-year journey. We have come an awfully long way, but before too long my three brothers and I will know much more about the approximate route our own people took.

1 comment:

  1. Everyone outside certain of the United States of America knows what natural selection is [...]This gave me a laugh. You obviously haven't spent any time in the Muslim world, they make American evangelicals look like amateurs in their creationistic dogma.